1. Field of the Invention
This invention relates to a method of coding a video signal for digital storage media and transmission media.
2. Description of the Prior Art
Discrete Cosine Transform (DCT) has been widely used for bandwidth compression of digital color video signals. It is the adopted basic method by the ISO/WG11 MPEG (Motion Picture Experts Group) for recommendation to the International Standardization Organization (ISO) for coded representation of moving picture information at around 1.15 Mbits/s data rate. In this proposed standard by MPEG, combination of motion compensation techniques and DCT coding method is used. The DCT coding method involves the discrete cosine transformation of the motion or non-motion compensated input block, the quantization of the DCT coefficients based on quantization matrixes and a rate-controller, zig-zag scanning of the quantized DCT coefficients, and variable length coding the run and level of the quantized DCT coefficients. The method proposed by MPEG is described in detail in the document "MPEG Video Simulation Model Three (SM3)," ISO-IEC/JTC1/SC2/WG8, MPEG 90/041, 1990.
The basic coding method of SM3 can be applied to various input video format, and adapted to code video signal at different data rates.
Basically, DCT converts statistically dependent picture elements (pixels) into independent coefficients. The results are usually energy concentrated into only a few of the coefficients containing the main part of the picture information which are quantized and run-length encoded. However, it is not easy to apply this concept when the transform pixel block is small in size and contains an edge boundary of an object. Transformation of such edge block will not lead to efficient compaction of signal energy. To illustrate this problem, two blocks of 8.times.8 pixels, one containing an edge, were extracted from an image sequence and two dimensionally discrete cosine transformed. The results are shown in Tables 1A, 1B, 2A and 2B.
TABLE 1A ______________________________________ Non-Edge Block Sample ______________________________________ 104 106 109 107 104 101 99 98 105 107 104 102 97 100 99 94 102 102 99 97 98 96 94 95 98 98 97 98 92 89 93 91 96 93 96 93 88 89 88 85 90 91 90 84 88 85 84 81 86 87 88 84 81 85 83 79 83 79 82 79 82 83 82 82 ______________________________________
TABLE 1B ______________________________________ Transformed Non-Edge Block Sample ______________________________________ 740 20 -1 -1 -4 2 1 1 60 6 -2 -3 1 0 -2 -2 0 -5 -2 1 -1 -1 2 0 3 4 -4 -1 -2 0 1 1 1 -3 -2 -3 3 3 1 1 /16 1 0 -2 -3 1 0 -1 0 -1 -2 0 1 3 -4 0 3 1 0 -1 0 0 0 3 -1 ______________________________________
TABLE 2A ______________________________________ Edge Block Example ______________________________________ 244 232 241 233 222 207 211 207 233 190 176 210 217 228 210 211 74 70 76 118 220 234 216 210 170 194 181 100 52 188 237 210 243 230 241 88 70 227 223 208 236 232 222 249 90 73 236 217 242 222 254 156 41 200 232 204 247 229 243 212 44 129 245 206 ______________________________________
TABLE 2B ______________________________________ Transformed Edge Block Sample ______________________________________ 1527 0 189 -56 -106 104 23 -28 20 -108 -134 117 76 -100 -1 28 111 78 -53 -24 58 32 -19 -7 109 133 24 -53 7 18 -7 5 27 63 63 -22 -70 4 26 0 /16 -38 -30 4 -11 -36 -1 10 -4 -41 -46 -58 -33 72 6 -67 17 -19 -60 -22 69 -35 -46 61 17 ______________________________________
From Tables 1A, 1B, 2A and 2B, it can be seen that with the non-edge block, the energy of the block after transformation is concentrated at the DC coefficient (top-left corner) and the two AC coefficients near it. On the other hand, the transformed edge block AC energy is randomly distributed. Coding of such transformed block is very difficult, and leads to ringing effects (or corona effects) around the edges usual to transform coding method.
Conventional DCT coding method is efficient in coding block with spatially correlated pixel values. However, if the block contains an edge boundary of an object against a smooth background, the spatial correlation of the pixel values in the block will be low and the two dimensional discrete cosine transformation of the block will not lead to efficient compaction of the block signal energy. The quantization process of the DCT coefficient of such block usually creates large distortion or quantization noise that cannot be masked.